1. Field of the Invention
The present invention relates an ultrasonic testing sensor and an ultrasonic testing method in which a sensor for transmission and a sensor for reception are used.
2. Description of the Related Art
First, a principle of a general phased array ultrasonic testing method (in the following description, ultrasonic testing is referred to as UT) is described. As illustrated in FIG. 8, ultrasonic elements (hereinafter referred to simply as elements) of a rectangular parallelepiped which configure a UT sensor are arrayed in parallel to each other. Ultrasonic wave outgoing starting time differences (hereinafter referred to as delay time periods) between the elements are adjusted so that ultrasonic waves arrive at the same time at a focus to raise the sound pressure at the focus to carry out testing or flaw detection. By detecting the delay time periods, the focal position is changed and an ultrasonic wave is scanned.
The limiting factor of the element pitch of the array sensor having the configuration described above is such as given below. When a main lobe (hereinafter referred to simply as ML) is converged to the focus, a grating lobe (hereinafter referred to simply as GL) along which the phases of the ultrasonic waves are aligned appears as depicted in FIG. 9. The generation angle Δφ of the GL with respect to the ML is represented by the following expression (1) (refer to, for example, page 3, line 16 of a document http://www.mlit.go.jp/chosahokoku/h16giken/h15/pdf/0502.pdf):2d·sin Δφ=n·λ  expression (1)where
d: element pitch [mm];
n: integer; and
λ: ultrasonic wavelength [mm].
Where a reflection source exists in an incident direction of the GL, a false signal which is recognized in error as a reflected wave of the ML. Therefore, The element pitch of the sensor is restricted to the range in which the GL represented by the following expression (2) is not generated:n·λ/2d=sin θ>1d<λ/2(n=1)  expression (2)
Meanwhile, as a method of improving the sensitivity of a phased array UT by sensor area expansion, a sensor having an ultrasonic element pitch equal to or greater than λ/2 is known (refer to, for example, JP-2009-293980-A). Further, an ultrasonic probe configured from a combination of a plurality of array sensors between which the installation angle with respect to a testing target is different is known (refer to, for example, JP-2013-42974-A).